The present invention relates generally to variable reluctance magnetic transducers and, in particular, to a passive-type magnetic transducer especially suited for sensing the speed of a rotating object.
Passive-type variable reluctance magnetic transducers are well known and have been widely used as speed sensors in electric control circuits for monitoring the speed of an associated rotating object such as, for example, a gear, a wheel or a bearing. Typically, the transducer includes a permanent magnet which is adjacent to one end of a pole piece formed from a ferrous material. An opposite end of the pole piece extends outwardly from the transducer to define a pole piece free end. The pole piece free end is adapted to be spaced from projecting ferrous elements attached to a rotating object, such as the teeth of a rotating gear. A coil surrounds the pole piece for sensing changes in magnetic flux through the coil. The coil is connected to generate an output signal to an associated electronic control circuit. An example of a prior art passive-type magnetic speed sensor can be found in U.S. Pat. No. 5,032,790.
In operation, a magnetic field extends from the magnet through the pole piece and out into the air space at the free end of the pole piece. The return path of the magnetic field is from the air space to the other end of the magnet. As a ferrous element approaches the tip of the pole piece, the reluctance of the magnetic circuit decreases, thus increasing the magnetic field. As the ferrous object passes away from the pole piece, the magnetic field decreases. When the magnetic field increases, it induces a voltage in the coil in one direction and, when it decreases, it induces a voltage in the coil in the opposite direction. The passage of one ferrous object (such as one gear tooth) induces one cycle of AC voltage. The AC voltage is proportional to the rate of change of magnetic flux in the magnetic circuit, and is generally proportional to the speed of the ferrous objects passing the pole piece, at least up to a predetermined speed. The frequency of the AC signal is directly proportional to the number of ferrous objects passing the pole piece per unit of time.
When a variable reluctance magnetic sensor is used, it is important to accurately fix the position of the face of the free end of the pole piece with respect to the trajectory of the periphery of the projecting ferrous element. In practice, it has been observed that an axial shift of the end of the pole piece by a few tenths of a millimeter in the direction of increasing the spacing separating the pole piece from the ferrous element can lead to a loss of useful signal.
According to the prior art, variable reluctance magnetic transducers are typical formed by inserting the permanent magnet and pole piece into a cylindrical cavity formed in a transducer core. The transducer core is usually molded from plastic material. As an alternative, the magnet and pole piece are placed in an end-to-end relationship and the core molded thereover. The alternate method allows use of a non-cylindrical pole piece. With both methods, the accuracy of the position of the free end of the pole piece with respect to the rotating ferrous object depends upon the cumulative tolerances of the length of the magnet and the length of the pole piece.